Far and away, I think Season Two is the best year. Every episode is extremely funny or it has a heartwarming story (back when the Simpson family acted like a family that loved each other). Classic episodes from that year are Treehouse of Horror, Bart the Daredevil, Itchy & Scratchy & Marge and The Way We Was.

Obviously, we're not getting a consensus here. Either way, I think almost every season from the first decade of the show has multiple undisputable classics.

Seasons 4 and 5 are nearly equally great, with 5 just slightly better, IMHO. Season 5 just has more of my personal favorites (like Rosebud, Homer Goes to College, Boy Scouts N Da Hood, and Homer Loves Flanders). But again, Season 4 is almost as good (with favorites Homer the Heretic and Marge vs. the Monorail). I think 4's when they really hit their stride, in fact. 6 is great, but not AS great and it went downhill (gradually) from there on out. 9 is when the bottom fell out (again, IMHO!). It still has it's great episodes but the seasons, collectively, just weren't the same. YMMV.

Season 8 is my favorite. It includes what I think are the two best episodes: "Homer's Phobia," where Homer thinks Bart might be gay, and "Homer's Enemy," which features the life and death of Frank Grimes (or Grimey, as he liked to be called).